feat: enhance parseRemoteAuthority to handle domains containing "--"

[ntoofu]

Problem

As explained in CONTRIBUTING.md,

The host name takes the format coder-vscode.----.

The parser splits on "--" to extract each component, but domain labels may contain "--".
In practice, IDNA/Punycode-encoded domain labels use xn-- as the ACE prefix.
So the naive split("--") misparses hostnames with such labels, and causes errors like the following image.

For example, a Coder deployment coder.xn--eckwd4c7cu47r2wf.jp would produce an SSH host of coder-vscode.coder.xn--eckwd4c7cu47r2wf.jp--yourname--your-workspace.
When split on "--", this yields ["coder-vscode.coder.xn", "eckwd4c7cu47r2wf.jp", "yourname", "your-workspace"] but the parser expects the first element to be the full hostname, so vscode-coder would try to open a workspace yourname whose owner is eckwd4c7cu47r2wf.jp and fails.

Solution

Added a reassembly step after the initial split("--").
The algorithm scans from the front of the split result:

• The first segment is always a part of a hostname.
• If the second segment contains a dot, that must be a fragmented part of a hostname. So merge those two segments back with "--".
• Because more than one domain labels may contain "--", repeat this merging process until a dot does not appear in the second segment.

This works because Coder usernames never contain dots,
so any dot-bearing segment after the split must be a hostname fragment.

Note: a TLD containing "--" would not be handled correctly because such hostname would fragment into segments the last of which does not contain dot, but no such TLD exists today.